Thats not really the issue for me.
All my clients know clearly that the 30 limit is in place, and all they have to do is request it be raised- the 30 limit is really just for new clients.
60 emails is still 1440 a day- enough to get the server blacklisted on spam databases if it should happen.

I don't know of any way to change the Exim error message and sort of suspect you won't be able to. You could search around on the Exim mailing lists to see if any gurus know about how to do this.

I know you don't like hearing this, but I also think 30 is too low. If you run csf it detects mail limits being hit and sends you a warning email, and you may also be able to get their IP blocked if they do hit their limits. As well as that, if you check your customers properly you won't have problems with spammers. 1440 emails per day is only barely enough to get you listed, in practice. You could have that many escape from a trusted user account (ie with higher mail limits) that had a contact script exploited, so I don't think the low limit actually gains you anything. I use a limit of 250 which means I only rarely have to raise it.